Diets of Atlantic Sharpnose Shark (Rhizoprionodon terraenovae) and Bonnethead (Sphyrna tiburo) in the northern Gulf of Mexico

Abstract

Diets of two coastal sharks, Atlantic Sharpnose Shark (Rhizoprionodon terraenovae) and Bonnethead (Sphyrna tiburo), were examined along the Texas and Alabama coasts in the northern Gulf of Mexico (GOM). Atlantic Sharpnose Sharks were collected from the northwest (n= 209) and northcentral (n= 245) GOM regions while Bonnetheads were collected from two locations within the northwest GOM (Galveston, Texas, n= 164; Matagorda, Texas, n= 79). Dietary analysis was conducted using stomach contents identified to the lowest taxonomic level, which were quantified using the index of relative importance (IRI) and non-parametric statistical analyses. Atlantic Sharpnose Sharks were revealed to be primarily piscivorous, with an overall %IRI of 79.76% for teleost fishes. Bonnetheads were shown to prey primarily on crustaceans (90.94% IRI), mainly crabs (22.06% IRI). Diets for Atlantic Sharpnose Sharks and Bonnetheads were evaluated by region and ontogeny, where variations by ontogeny were examined based on length at 50% maturity (L 50 ) values, delineating mature from immature individuals. Atlantic Sharpnose Sharks and Bonnetheads showed a decrease in dietary prey species richness from juveniles to adults using %IRI. Regional dietary differences existed with Atlantic Sharpnose Sharks from the northwest GOM consuming more crustaceans than conspecifics from the northcentral GOM. Bonnetheads collected from Galveston, TX consumed more crab than Bonnetheads from Matagorda, TX, while Bonnetheads from Matagorda, TX displayed a diet with higher prey species richness. Our results highlight differences in diets of two common shark species at both local and regional spatial scales.

Observations on heterodonty within the dentition of the Atlantic Sharpnose Shark, Rhizoprionodon terraenovae (Richardson, 1836), from the north-central Gulf of Mexico, USA, with implications on the fossil record

“…The data provided by Drymon, Powers & Carmichael (2012) and Harrington et al (2016) showed a relationship between the percentage of fishes in the stomach and the length of a shark, with the mean weight (%W) and number (%N) of fishes generally increasing as the shark matured ( Fig. 17 , Appendices 1.35 – 1.36 ).…”

“…Various studies have shown that the diets of R. terraenovae populations in the northern Gulf of Mexico differ slightly by region as well as ontogenetic stage ( Bethea, Buckel & Carlson, 2004 ; Bethea et al, 2006 ; Drymon, Powers & Carmichael, 2012 ; Harrington et al, 2016 ; Plumlee & Wells, 2016 ; Seubert et al, 2019 ). In the northern Gulf of Mexico, young-of-the-year individuals primarily feed on shrimp, crustaceans, and cephalopods, but they become increasingly reliant on fishes (mainly sciaends and clupeids) through ontogeny.…”

“…In the northern Gulf of Mexico, young-of-the-year individuals primarily feed on shrimp, crustaceans, and cephalopods, but they become increasingly reliant on fishes (mainly sciaends and clupeids) through ontogeny. We compared our data to the datasets utilized by Drymon, Powers & Carmichael (2012) and Harrington et al (2016) to determine if there was a relationship between the reported ontogenetic dietary shifts for R. terraenovae and the ontogenetic tooth stages we observed in our sample. Drymon, Powers & Carmichael (2012) and Harrington et al (2016) used both nitrogen and carbon stable isotope ratios and stomach content analyses to examine the diets of 454 male and female R. terraenovae specimens across various growth stages caught from the northern Gulf of Mexico between Alabama and Texas, USA.…”

“… STL, stretched total length. Data derived from Drymon, Powers & Carmichael (2012) and Harrington et al (2016) . …”

“…Consequently, much is known about the distribution ( Drymon et al, 2010 ; Bethea et al, 2014 ), movement ( Gurshin & Szedlmayer, 2004 ; Carlson et al, 2008 ), age and growth ( Carlson & Baremore, 2003 ), reproduction ( Parsons, 1983 ; Hoffmayer et al, 2013 ), sexual segregation ( Drymon et al, 2020 ), and population structure ( Davis, Suárez-Moo & Daly-Engel, 2019 ) of this species. The dietary habits of Atlantic Sharpnose Sharks have been particularly well studied and demonstrate that the trophic niche of this species varies spatially ( Drymon, Powers & Carmichael, 2012 ; Plumlee & Wells, 2016 ; Seubert et al, 2019 ) and ontogenetically ( Bethea, Buckel & Carlson, 2004 ; Bethea et al, 2006 ; Harrington et al, 2016 ). Atlantic Sharpnose Sharks of the north-central Gulf of Mexico shift from an invertebrate-rich diet as juveniles to one consisting primarily of fish as adults, with a concurrent increase in prey size as individual body size increases.…”